Headphones are typically located in, on or over the ears. One result is that outside sound is occluded. This has an effect on the wearer's ability to participate in conversations as well as the wearer's environmental/situational awareness. It is thus desirable at least in some situations to allow outside sounds to reach the ears of a person using headphones.
Headphones can be designed to sit off the ears so as to allow outside sounds to reach the wearer's ears. This type of headphone is sometimes referred to as an open headphone. Two benefits of an open headphone are situational awareness and being un-occluded.
The value of these benefits diminishes as the external environment starts getting noisier and the user is not able to enjoy the audio that they are listening to. In noisy environments above, for example, 70 dBA (especially babble), the open headphone experience deteriorates rapidly. It is in these environments that the open headphone can benefit from active noise reduction (ANR).